Reporting similar problem with plasma-desktop and Fedora 15:
If Dell Laptop is left on overnight plasma-desktop is consuming ~92% CPU in the morning. The following
kquitapp plasma-desktop && sleep 10s && plasma-desktop &
brings the CPU usage down to ~3% and the system okay for the rest of the work day.
When the plasma-desktop is using high CPU, there is plenty of RAM free (~500M) and disc activity is negligible. Had been using Fedora 13 since its release with no such problems.
One thing that is different using Fedora 15 on my system is the behaviour of the external monitor. With the laptop lid closed the ATI Mobility Radeon x600 seems to be driving a resolution of 1920x1200 to the external monitor and some of the display is then not physically viewable. My work around is to tell "Configure Display" that the laptop LVDS display is 1600x1200 (which it is not, it is actually 1920x1200 but the lid is closed so shouldn't hurt anything right?). Then the 1600x1200 external monitor displays correctly. The only other thing that is weird about my system and Fedora 15 is that the login manager does not display on the external monitor - the wallpaper is there, just no login field. I blindly type in my password, hit enter, do the "Configure Display" hocus pocus and I'm good to work for several hours with performance not too much worse than Fedora13.

I have this one too. At least part of the problem is using the Network Manager to monitor bandwidth.
1: Run "top" in a shell window. Observe that plasma-desktop is quiescent.
2: In the System Tray click the Network Monitor, then click an active interface to show the bandwidth graph.
3: Observe on "top" that plasma-desktop is now consuming 100% of a CPU (on multi-core systems this will be shown as a percentage of total CPU capacity). This continues even when the System Tray bandwidth graph is not visible.
4: Click the "Go Back" arrow at the top of the network interface details to close the bandwidth graph.
5: Observe on "top" that the plasma-desktop is quiescent once more.

This bug appears to have been reported against 'rawhide' during the Fedora 19 development cycle.
Changing version to '19'.
(As we did not run this process for some time, it could affect also pre-Fedora 19 development
cycle bugs. We are very sorry. It will help us with cleanup during Fedora 19 End Of Life. Thank you.)
More information and reason for this action is here:
https://fedoraproject.org/wiki/BugZappers/HouseKeeping/Fedora19

Hmm, so I "fixed" the issue by wiping plasma cfg out and letting plasma-desktop to create new one (and spending some time to configure it according my taste again). Probably the cause was some mad applet - don't know which as now I use the same set of widgets with no problem.
However, I kept the old configuration for the purpose of testing, if needed.

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